1. Field of the Invention
The present invention relates to image detectors of the type using arrays of photosensitive sensors made from semiconductor materials. The invention applies in a particularly beneficial manner (but not exclusively) in the case of the detection of radiological images. Its aim is to facilitate the manufacture and to reduce the costs thereof, as well as to improve the stability of the measurements and the quality of the images obtained, in the case in particular of image detectors using a large number of photosensitive sensors.
2. Discussion of the Background
It is common practice to use arrays of sensors or photosensitive points made of semiconductor materials, in image acquisition techniques. The photosensitive points are made on semiconductor material, silicon for example, and they very often each comprise at least one photodiode. The photodiodes are sensitive in a band of wavelengths generally corresponding to visible radiation or near-visible radiation.
Depending on the applications for which they are intended, the photosensitive arrays may comprise highly variable numbers of pixels, that is to say of photosensitive points, from a few to several tens of thousands (and possibly as many as several millions of photosensitive points for dimensions of the order of for example 50 cmxc3x9750 cm).
One of the benefits of the images obtained with the aid of photosensitive points made of a semiconductor material lies in the fact that these images can be digitized, offering as advantages in particular ease of processing and of storage of the image. Of course, the advantages connected with images of digital type are just as important in the field of radiological image detection, and particularly in that of medical imaging using X-rays.
To apply photosensitive arrays such as described above to the detection of radiological images, it is well known to interpose a screen made of a scintillator substance between the X-ray radiation and each of the photosensitive points of the array. The scintillator substance is chosen so as to convert the X-ray radiation into light radiation, in the wavelength band to which the photosensitive points are sensitive.
According to one of the common operating modes, each photosensitive point comprises an element which functions as a switch, mounted in series with the photodiode. The line-by-line control of each of the switch elements (control effected after an exposure phase or integration phase, in the course of which the photodiode is exposed to a measurement light signal, that is to say to a signal corresponding to an image to be detected), makes it possible to transfer to a column electric charges produced by the corresponding photodiode during the integration phase; it should be noted that during the integration phase, the photodiode is fairly strongly reverse biased, so that it forms a capacitance in which the charges generated during this phase are stored.
Amplifiers and multiplexing circuits then make it possible to transfer the charges from the various photosensitive points to a readout and data processing circuit. An array each of whose photosensitive points comprises a photodiode cooperating with a switch element, as indicated above, is described together:with its operating mode in a French patent application No. 86.14.058 (publication No. 2,605,166).
In the operating example mentioned above, the charges are stored, as and when they are generated, in the zone of the photodiode by which they were produced. According to another fairly common operating mode, both as regards X-ray imaging and imaging based on visible radiation, the charges are transferred as soon as they are produced, so as to be stored outside the zone of the photodiode, for example at the level of an amplifier catering for an integrator function. This configuration, which may be encountered for example in X-ray CT scanners, poses in particular various problems which the invention aims to solve. However, it should nevertheless be noted that the solutions proposed by the invention find application also in the operating modes mentioned above.
CT scanners are X-ray apparatus which generally use a single source of X-rays and a detector assembly which may comprise a large number of photosensitive points. The assembly formed by the source and the detector assembly can rotate and/or move in translation with respect to the body of a patient, with a view to forming the internal image of a slice of the patient. Such an apparatus is described for example in the document U.S. Pat. No. 5 592 523.
FIG. 1 diagrammatically and in a simplified manner represents some of the essential elements of a CT scanner. The CT scanner comprises a source 1 producing X-ray radiation 2, and a detector assembly 4. The X-ray radiation 2 irradiates a body 3 of a patient interposed between the source 1 and the detector device 4. The CT scanner rotates about an axis of rotation represented by a point 6. It may furthermore comprise an auxiliary detector 5, situated outside the X-ray field masked by the body 3. The detector device 4 has a length which extends according to an arc of a circle. The detector assembly 4 comprises a multitude of photosensitive detection points arranged along the length and the width of the detector device 4. The photosensitive points may be grouped together into detection modules MD.
A CT scanner can comprise for example up to several tens of detection modules, arranged side by side along the length of the detector device 4. Each detection module MD comprises a scintillator material 71 superimposed on a photosensitive array 72. The scintillator material has the function of converting the X-ray radiation into light radiation to which the photosensitive arrays are sensitive; the scintillator material is therefore situated on the source 1 side.
A photosensitive array 72 can comprise a grid of photodiodes (not represented in FIG. 1), arranged along for example 32 rows and 16 columns, i.e. 512 diodes in number. In certain applications such as those relating to CT scanners, each photodiode is linked to a data acquisition amplifier which has the function in particular of receiving and integrating the charges as and when they are produced by the photodiode; this integration of the charges achieves in a certain manner the storage of the charges which, in the other operating mode mentioned previously, is performed at the level of the photodiode itself, whereas in the present case this storage is performed outside the photosensitive array.
In fact in this configuration the amplifiers which receive the signals from the photodiodes are not made on the same substrate as these latter, these amplifiers may be located in proximity to the photosensitive array.
FIG. 2 diagrammatically represents a photosensitive array 7 similar to an array 72 of FIG. 1, and each photosensitive point of which is formed by a photodiode. To simplify the description only four photodiodes Dp1 to Dp4 are represented. In the example, they are arranged in two rows L1, L2, and two columns cl1, cl2. The photodiodes Dp1 to Dp4 are made on a substrate Sb1 and, as a constructional example, they are all linked electrically by one and the same end, their anode in the non-limiting example represented, to the said substrate and to ground.
The other end of each of the photodiodes, i.e. the cathode, is connected to an individual conductor 19, by way of which each of these cathodes is connected to a first input E1 of an amplifier a1 to a4 specific to each photodiode.
The amplifiers a1 to a4 are commonly constructed with the aid of operational amplifiers; however, it is also known practise to embody them as discrete components, with one or more transistors. These amplifiers intended for acquiring the data delivered by the photodiodes (in the form of electric charges), must meet very severe constraints: low noise, large dynamic range and high stability for the reproduction of the measurements.
In the conventional example represented, each first input E1 in fact corresponds to the xe2x80x9cxe2x88x92xe2x80x9d inverting input of the operational amplifier a1 to a4.
A so-called integration capacitance Ci is linked between the first input and an output S1 of each amplifier a1 to a4. A first switch element I1 (consisting for example of a transistor of the MOS type) is mounted in parallel with each integration capacitance Ci. A second input Ep of the amplifiers a1 to a4 (corresponding to the xe2x80x9c+xe2x80x9d noninverting input of the amplifier), called the xe2x80x9cdrive inputxe2x80x9d, receives a voltage called the drive voltage VP1, which in the example is developed with respect to ground.
The application of the drive voltage VP1 to a drive input Ep has the effect of establishing on the first electrode E1 a voltage which constitutes the bias voltage VP of the photodiodes Dp1 to Dp4. The value of the bias voltage VP is therefore dependent on the value of the drive voltage VPl whose variations it follows, and to which it more or less closely approximates depending in particular on the nature and the qualities of the amplifier. This shows that each of the amplifiers used to receive the charges delivered by photodiodes Dp1 to Dp4 must comprise, in addition to the input E1 linked to the photodiode, another input such as the drive input Ep so as to receive the drive voltage VP from which the bias potential VP present on the first input E1 stems.
The output S1 of each of the amplifiers al to a4 is linked to a second switch element I2, whose other side is linked to a so-called storage capacitance Cs. The other plate of the storage capacitance Cs is linked in the example to ground.
Various other means are also linked to the storage capacitances Cs, which make it possible in particular to transfer the charges to multiplexing and data processing circuits, but these means are not described further since they depart from the field of the invention and are known per se. In fact, the region directly concerned by the invention stops at the output S1 of the amplifiers a1 to a4.
During the exposure or integration phase, the first and second switches I1, I2 are open, and the charges delivered by the photodiodes Dp1 to Dp4 ate integrated in the corresponding amplifier a1 to a4. In a following phase, the second switches I2 are closed, and the quantities of charge integrated by the amplifiers during the integration phase are transferred to the storage capacitances Cs; when this transfer is effected the second switches I2 are placed in the open state, then the first switches I1 are closed for a short instant so as to discharge the integration capacitances Ci and allow a new cycle.
For correct operation of the photodiodes Dp1 to Dp4 in the integration phases, during which phases they deliver charges whose quantity is dependent on the intensity of the light signal to which they are exposed, the bias voltage applied to these photodiodes must generally lie between 0 volt (zero) and a reverse bias value of 20 mV. This bias must be stable and applied equally to all the photodiodes, if the quality of the measurements and their stability is not to be seriously impaired.
In the case of apparatus which use a lowish number of photodiodes, a few tens for example, it may be acceptable to process the signals delivered by these photodiodes by using an individual channel for each, and by making any adaptations required for the proper operation of each channel: for example by adjusting for each amplifier the value of the drive voltage which it receives, so that each photodiode is correctly biased.
However, in the case of arrays which may comprise several hundreds of photodiodes, this method is no longer suitable and the integration of the amplifiers such as a1 to a4 becomes necessary, as regards ease of manufacture, bulkiness, heat dissipation and overall cost. Such integration unfortunately poses problems regarding its compatibility with the photosensitive array: specifically, if one no longer resorts to individual processing channels, it becomes more difficult to use for example operational amplifiers for reasons in particular of differences of voltage threshold, as is further explained in the subsequent description.
It should be noted that we intend the term xe2x80x9cintegrationxe2x80x9d to imply the use (and possibly the embodying) of an integrated circuit containing in a manner which is conventional per se, at least all the components required to form, on one and the same substrate, what could become a relatively sizeable number of amplifiers such as a1 to a4, numbering 61, 128, 256 or 512 or more for example.
It should also be noted that the amplifiers a1 to a4 fulfil several functions including that of amplifier, plus that of integrator, and plus that which consists in applying one of the potentials of the bias voltage VP to one of the ends of the photodiode linked thereto. Also, these amplifiers are called adapter amplifiers a1 to a4 in the subsequent description.
The fact that the photodiodes Dp1 to Dp4 are embodied on a substrate and that the adapter amplifiers a1 to a4 are on another substrate, poses a difficulty as regards stability in relation to temperature variations, since the thermal drifts in this case may be very different and may be injurious. Specifically, the threshold voltage of a transistor varies fairly strongly with temperature, and tends therefore to modify the difference between the voltage values present for example on the first inputs E1 and the drive inputs Ep of the adapter amplifiers a1 to a4.
Moreover, it is known that integrated circuits originating from different manufacturing batches have different operating voltages. In this case they impose different bias voltages on the photodiodes associated with them.
It is known practise to use amplifiers of differential type (thermal drift of less than 20 xcexcV/xc2x0C.) when seeking low thermal drift and good stability in general. From this standpoint, differential amplifiers could therefore constitute the sought-after integrated amplifiers, but in the case of the present application, this type of amplifier may be inadvisable owing in particular to the fact that its input stage requires two transistors, and that it thus tends to raise the noise by a relatively sizeable factor {square root over (2)} (square root of two). Furthermore, an amplifier of this type uses twice the area of silicon, thereby tending to increase its bulk and its cost.
Another solution (favourable to low noise) can also consist in using an adapter amplifier which uses just one input transistor.
FIG. 3 represents the diagram of an amplifier axe2x80x2 capable of fulfilling the functions catered for by the adapter amplifiers a1 to a4 of FIG. 2.
The amplifier axe2x80x2 comprises three MOS type transistors, Q1, Q2, Q3. The transistors Q1 and Q2 form a well-known cascode type transconductance amplifier. The transistor Q1 can comprise a large area which endows it with low voltage noise on its gate G. This gate G of Q1 is linked to the source S of the third transistor Q3 and this point forms an input E of the amplifier axe2x80x2 ; this input E with respect to the amplifiers a1 to a4 shown in FIG. 2, represents the xe2x80x9cxe2x88x92xe2x80x9d inverting input and therefore represents a first input such as E1 to which a photodiode Dp (represented dotted) is to be linked. The drain D of Q1 is linked to the source S of the transistor Q2 whose gate G is held at a fixed voltage VF, and whose drain D is linked on the one hand to a supply voltage V+ by way of a current generator Gi, and on the other hand to the gate G of the third transistor Q3. The drain D of Q3 receives one end of an integration capacitance Cixe2x80x2 whose other end is linked to the supply voltage V+.
The capacitance Cixe2x80x2 integrates the currents and a switch I1xe2x80x2 arranged in parallel with the capacitance Cixe2x80x2 makes it possible to reset the latter""s charge to zero periodically. The drain D of Q3 constitutes the output of the amplifier axe2x80x2 and corresponds to an output S1 of the amplifiers of FIG. 2 (this output would therefore have to be connected to a second switch I2 shown in FIG. 2). The source S of the first transistor Q1 constitutes a second input Ep of the amplifier axe2x80x2 ; this input Ep corresponds to the xe2x80x9c+xe2x80x9d noninverting inputs of the amplifiers of FIG. 2, and it therefore constitutes a drive input intended for receiving a drive voltage VP1, so as to determine on the input E1 the bias voltage VP2.
Although it exhibits advantages in particular as regards noise, as compared with a differential amplifier, the amplifier diagram described above also poses problems, if it is chosen so as to be integrated as envisaged above. This amplifier is in fact subject to a considerable thermal drift of the order of 2 mV/xc2x0C. Moreover, when integrating this type of amplifier, discrepancies of from 0.1 volts to 0.2 volts are possible from one integrated circuit to another. Consequently, if several such integrated circuits are associated, each circuit will have its own threshold voltage. So, one seeks a bias voltage of the photodiodes of between 0 and 20 millivolts.
These explanations show that, although it is desirable, the integration mentioned above is especially difficult to implement.
The aim of the present invention is to solve the problems cited above, and in particular those related to one and the same bias for all the photodiodes used as well as to the maintaining of this bias over time, and hence to allow the integration of the elements useful in the acquisition of data of photosensitive arrays, and to permit the use of a wide selection of amplifier type.
For this purpose the invention provides for the making of adapter amplifiers made in the form of integrated circuits on one and the same substrate, and for the use of one of these integrated adapter amplifiers to obtain a potential acting as reference, so as to define the bias voltage of the photodiodes with respect to this reference potential.
This combination makes it possible to solve all or some of the problems posed, since in practise the amplifiers of one and the same integrated circuit have the same thermal variation to within a few fractions of an mV.
The invention therefore relates to an image detector, comprising at least one photodiode array, adapter amplifiers intended for receiving electric charges produced by the photodiodes, these charges being produced during a so-called integration phase during which the photodiodes are each biased by a bias voltage, the bias voltage being applied to each photodiode by way of an adapter amplifier, characterized on the one hand in that the adapter amplifiers are constructed as at least one integrated circuit, and in that it furthermore includes means for defining the bias voltage with respect to a reference potential tapped off from an adapter amplifier.